Move instead of deep-copy ordered_json values when an object grows

docs/mkdocs/docs/api/ordered_map.md

@@ -28,6 +28,11 @@ A minimal map-like container that preserves insertion order for use within [`nlo
 The type uses a `std::vector` to store object elements. Therefore, adding elements can yield a reallocation in which
 case all iterators (including the `end()` iterator) and all references to the elements are invalidated.
 
+When the storage grows, the keys are copied and the mapped values are moved to the new storage. A plain `std::vector`
+would copy the whole elements instead, because their `#!cpp const` keys make them not nothrow move constructible; for
+[`ordered_json`](ordered_json.md), this would be a deep copy of every nested value. The values are only copied if
+`T` is not default constructible or not nothrow move assignable.
+
 ## Member types
 
 - **key_type** - key type (`Key`)
@@ -56,6 +61,11 @@ std::equal_to<>     // since C++14
 - **find**
 - **insert**
 
+## Exception safety
+
+**emplace**, **operator\[\]**, and **insert(value)** have the strong exception guarantee: if an exception is thrown (for
+instance, because copying a key or allocating memory fails), the contents of the container are unchanged.
+
 ## Complexity
 
 Because the elements are stored in a `std::vector` in insertion order, there is no index to look a key up by. Every
@@ -122,3 +132,4 @@ This differs from `#!cpp std::map`, where the same operations are O(log n).
 
 - Added in version 3.9.0 to implement [`nlohmann::ordered_json`](ordered_json.md).
 - Added **key_compare** member in version 3.11.0.
+- Changed in version 3.13.0: growing the storage moves the mapped values instead of copying them.

include/nlohmann/ordered_map.hpp

@@ -8,13 +8,15 @@
 
 #pragma once
 
+#include <algorithm> // max, min
 #include <functional> // equal_to, less
 #include <initializer_list> // initializer_list
 #include <iterator> // input_iterator_tag, iterator_traits
 #include <memory> // allocator
 #include <stdexcept> // for out_of_range
-#include <type_traits> // enable_if, is_convertible
-#include <utility> // pair
+#include <tuple> // forward_as_tuple
+#include <type_traits> // enable_if, integral_constant, is_convertible, is_nothrow_move_constructible
+#include <utility> // forward, move, pair, piecewise_construct
 #include <vector> // vector
 
 #include <nlohmann/detail/macro_scope.hpp>
@@ -79,7 +81,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(key, std::forward<T>(t));
+        append(key, std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -94,7 +96,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(std::forward<KeyType>(key), std::forward<T>(t));
+        append(std::forward<KeyType>(key), std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -368,7 +370,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::push_back(value);
+        append(value);
         return {--this->end(), true};
     }
 
@@ -386,6 +388,64 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
     }
 
 private:
+    /*!
+    @brief add an element whose key is not yet contained at the end
+
+    A std::vector copies all elements when it grows, because their const keys
+    make them not nothrow move constructible. For ordered_json, this is a deep
+    copy of every value. Where the strong exception guarantee can be kept, grow
+    the storage here instead, copying only the keys and moving the values.
+    */
+    template<typename... Args>
+    void append(Args&& ... args)
+    {
+        // evaluated here rather than at class scope, because T is still
+        // incomplete when basic_json instantiates its object_t
+        using move_values = std::integral_constant<bool, detail::conjunction<
+                detail::negation<std::is_nothrow_move_constructible<value_type>>,
+                std::is_copy_constructible<key_type>,
+                detail::is_default_constructible<mapped_type>,
+                std::is_nothrow_move_assignable<mapped_type>>::value>;
+        append_impl(move_values{}, std::forward<Args>(args)...);
+    }
+
+    template<typename... Args>
+    void append_impl(std::true_type /*unused*/, Args&& ... args)
+    {
+        if (this->size() < this->capacity())
+        {
+            Container::emplace_back(std::forward<Args>(args)...);
+            return;
+        }
+
+        // 1. May throw, but only changes tmp: copy the keys, value-initialize
+        //    the values, and add the new element. The arguments may refer to
+        //    elements of this container, so they are used before any value is
+        //    moved out of it.
+        Container tmp(this->get_allocator()); // equal allocators, so swap() is valid
+        tmp.reserve((std::min)(this->max_size(), (std::max)(size_type{1}, 2 * this->size())));
+        for (const auto& element : *this)
+        {
+            tmp.emplace_back(std::piecewise_construct, std::forward_as_tuple(element.first), std::forward_as_tuple());
+        }
+        tmp.emplace_back(std::forward<Args>(args)...);
+
+        // 2. Cannot throw: move the values over and adopt the new storage.
+        auto it = tmp.begin();
+        for (auto& element : *this)
+        {
+            it->second = std::move(element.second);
+            ++it;
+        }
+        Container::swap(tmp);
+    }
+
+    template<typename... Args>
+    void append_impl(std::false_type /*unused*/, Args&& ... args)
+    {
+        Container::emplace_back(std::forward<Args>(args)...);
+    }
+
     JSON_NO_UNIQUE_ADDRESS key_compare m_compare = key_compare();
 };
 

single_include/nlohmann/json.hpp

@@ -25768,13 +25768,15 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
+#include <algorithm> // max, min
 #include <functional> // equal_to, less
 #include <initializer_list> // initializer_list
 #include <iterator> // input_iterator_tag, iterator_traits
 #include <memory> // allocator
 #include <stdexcept> // for out_of_range
-#include <type_traits> // enable_if, is_convertible
-#include <utility> // pair
+#include <tuple> // forward_as_tuple
+#include <type_traits> // enable_if, integral_constant, is_convertible, is_nothrow_move_constructible
+#include <utility> // forward, move, pair, piecewise_construct
 #include <vector> // vector
 
 // #include <nlohmann/detail/macro_scope.hpp>
@@ -25841,7 +25843,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(key, std::forward<T>(t));
+        append(key, std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -25856,7 +25858,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::emplace_back(std::forward<KeyType>(key), std::forward<T>(t));
+        append(std::forward<KeyType>(key), std::forward<T>(t));
         return {std::prev(this->end()), true};
     }
 
@@ -26130,7 +26132,7 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
                 return {it, false};
             }
         }
-        Container::push_back(value);
+        append(value);
         return {--this->end(), true};
     }
 
@@ -26148,6 +26150,64 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
     }
 
 private:
+    /*!
+    @brief add an element whose key is not yet contained at the end
+
+    A std::vector copies all elements when it grows, because their const keys
+    make them not nothrow move constructible. For ordered_json, this is a deep
+    copy of every value. Where the strong exception guarantee can be kept, grow
+    the storage here instead, copying only the keys and moving the values.
+    */
+    template<typename... Args>
+    void append(Args&& ... args)
+    {
+        // evaluated here rather than at class scope, because T is still
+        // incomplete when basic_json instantiates its object_t
+        using move_values = std::integral_constant<bool, detail::conjunction<
+                detail::negation<std::is_nothrow_move_constructible<value_type>>,
+                std::is_copy_constructible<key_type>,
+                detail::is_default_constructible<mapped_type>,
+                std::is_nothrow_move_assignable<mapped_type>>::value>;
+        append_impl(move_values{}, std::forward<Args>(args)...);
+    }
+
+    template<typename... Args>
+    void append_impl(std::true_type /*unused*/, Args&& ... args)
+    {
+        if (this->size() < this->capacity())
+        {
+            Container::emplace_back(std::forward<Args>(args)...);
+            return;
+        }
+
+        // 1. May throw, but only changes tmp: copy the keys, value-initialize
+        //    the values, and add the new element. The arguments may refer to
+        //    elements of this container, so they are used before any value is
+        //    moved out of it.
+        Container tmp(this->get_allocator()); // equal allocators, so swap() is valid
+        tmp.reserve((std::min)(this->max_size(), (std::max)(size_type{1}, 2 * this->size())));
+        for (const auto& element : *this)
+        {
+            tmp.emplace_back(std::piecewise_construct, std::forward_as_tuple(element.first), std::forward_as_tuple());
+        }
+        tmp.emplace_back(std::forward<Args>(args)...);
+
+        // 2. Cannot throw: move the values over and adopt the new storage.
+        auto it = tmp.begin();
+        for (auto& element : *this)
+        {
+            it->second = std::move(element.second);
+            ++it;
+        }
+        Container::swap(tmp);
+    }
+
+    template<typename... Args>
+    void append_impl(std::false_type /*unused*/, Args&& ... args)
+    {
+        Container::emplace_back(std::forward<Args>(args)...);
+    }
+
     JSON_NO_UNIQUE_ADDRESS key_compare m_compare = key_compare();
 };
 

tests/benchmarks/src/benchmarks.cpp

@@ -119,6 +119,39 @@ BENCHMARK_CAPTURE(ParseIndented, canada / 4,       TEST_DATA_DIRECTORY "/nativej
 BENCHMARK_CAPTURE(ParseIndented, citm_catalog / 4, TEST_DATA_DIRECTORY "/nativejson-benchmark/citm_catalog.json", 4);
 BENCHMARK_CAPTURE(ParseIndented, twitter / 4,      TEST_DATA_DIRECTORY "/nativejson-benchmark/twitter.json",      4);
 
+//////////////////////////////////////////////////////////////////////////////
+// parse JSON from string into an ordered_json
+//
+// Same as ParseString above, but with nlohmann::ordered_json, whose objects
+// keep their members in a vector: the pair of rows shows what preserving the
+// insertion order costs.
+//////////////////////////////////////////////////////////////////////////////
+
+static void ParseStringOrdered(benchmark::State& state, const char* filename)
+{
+    std::ifstream f(filename);
+    std::string str((std::istreambuf_iterator<char>(f)), std::istreambuf_iterator<char>());
+
+    while (state.KeepRunning())
+    {
+        state.PauseTiming();
+        auto* j = new nlohmann::ordered_json();
+        state.ResumeTiming();
+
+        *j = nlohmann::ordered_json::parse(str);
+
+        state.PauseTiming();
+        delete j;
+        state.ResumeTiming();
+    }
+
+    state.SetBytesProcessed(state.iterations() * str.size());
+}
+BENCHMARK_CAPTURE(ParseStringOrdered, jeopardy,     TEST_DATA_DIRECTORY "/jeopardy/jeopardy.json");
+BENCHMARK_CAPTURE(ParseStringOrdered, canada,       TEST_DATA_DIRECTORY "/nativejson-benchmark/canada.json");
+BENCHMARK_CAPTURE(ParseStringOrdered, citm_catalog, TEST_DATA_DIRECTORY "/nativejson-benchmark/citm_catalog.json");
+BENCHMARK_CAPTURE(ParseStringOrdered, twitter,      TEST_DATA_DIRECTORY "/nativejson-benchmark/twitter.json");
+
 //////////////////////////////////////////////////////////////////////////////
 // serialize JSON
 //////////////////////////////////////////////////////////////////////////////

tests/src/unit-disabled_exceptions.cpp

@@ -46,6 +46,24 @@ TEST_CASE("Tests with disabled exceptions")
         CHECK(*sax_no_exception::error_string == "[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - invalid literal; last read: 'x'");
         delete sax_no_exception::error_string; // NOLINT(cppcoreguidelines-owning-memory)
     }
+
+    SECTION("growing an ordered_json object")
+    {
+        auto j = nlohmann::ordered_json::object();
+        for (int i = 0; i < 100; ++i)
+        {
+            j[std::to_string(i)] = {{"nested", i}};
+        }
+
+        CHECK(j.size() == 100);
+        int i = 0;
+        for (const auto& element : j.items())
+        {
+            CHECK(element.key() == std::to_string(i));
+            CHECK(element.value()["nested"] == i);
+            ++i;
+        }
+    }
 }
 
 DOCTEST_GCC_SUPPRESS_WARNING_POP